1. Field of the Invention
This invention relates generally to SONET based data transfer, and, more particularly, to the efficient and flexible interfacing of a SONET framer to a SERDES device.
2. Description of the Related Art
The growth in computer information technology has created a need to transfer data in an efficient and intelligent manner. One form of data transfer typically utilized is over a synchronous optical network (SONET). SONET is the American National Standards Institute for synchronous data transmission on optical media, and is a standard for connecting fiber-optic transmission systems. SONET defines a base data transfer rate of 51.84 Mbps and a set of multiples of the base transfer rate known as Optical Carrier Levels (OCx). Therefore, SONET typically establishes Optical Carrier Level from 51.8 Mbps to transfer speeds of over 9.953 gigabits per second (GPS).
The SONET standard enables data streams at different rates to be multiplexed. Multiplexing is generally utilized to combine multiple signals such as, for example, several lower rate signals into one higher rate signal for transmission over a single line or a media. One common type of multiplexing combines more than one low-speed signals for transmission over one high-speed connection. Prior rate standards used by numerous countries were often not compatible for multiplexing. With the actuation of SONET, communication carriers in many parts of the world can interconnect their digital carrier and fiber optic systems.
In addition, SONET may serve as the media upon which asynchronous transfer mode may run. Asynchronous transfer mode (ATM) is a switching technology that organizes digital data into 53-byte units and transmits them over a medium using digital signals. Typically, a cell may be processed asynchronously relative to other cells and is queued before being multiplexed over the transmission path.
Generally, systems or networks with data streams of different size and speed need to have their data converted to another type of data stream to be compatible with a another system. For example, in an optical data transmission system such as SONET, there may be a need to interface 622.08 MHz SERDES device to a 77.76 MHz Framer device. In an exemplary network, nodes are connected to other nodes by way of line cards within each node. In one example, a line card within each node has an optical input and an optical output that connects with another line card within other nodes. In this circumstance, a line card with a data stream converter utilizing PLL may be used to convert a 622.08 MHz 4 bit data transmission to a 32 bit 77.76 MHz data transmission and vice versa. A data stream converter utilizing a phase locked loop (PLL) system can change the clocking of data transfer frequencies to compensate for differences or variances in clock synchronization in different types of data transmissions. For example, due to differences in temperature or some other condition that can slightly alter data transmission speeds or clock speeds, input of data into the data stream converter may not be synchronized correctly with the data stream output. Therefore, for example, more data may be inputted into the converter than is being outputted and therefore, either loss of data or scrambling of data may occur. Also, the clock may be slower because of a small jitter, or phase shifts may occur in clocking of data transmission. PLL can determine these types of problems in the phase delay and once the phase variance is locked in, the PLL may find a phase that works with the phase variance. Therefore, PLL may alter the phase of the clocking of data input and output to compensate for many phase changes.
Unfortunately, there are great difficulties associated with using PLL systems. Because PLL is an analog system and SONET Framers are generally digital systems, problems that typically occur when using a combination of analog and digital components make the utilization of the PLL system more unreliable and difficult. In addition it is not easy to integrate an analog component with a digital component. For example, if an analog PLL system is utilized within a digital SONET Framer device, the digital SONET system may be subject to harmful signal noise generated by the analog system. Further problems may occur with validation of the chip. The component is often modeled to make sure it is working up to specification. Regrettably, analog components are generally difficult to model accurately while components utilizing pure digital simulation is usually much easier to model and therefore to validate proper working specifications. In addition to data validation being more difficult with analog components, more engineering, interfacing and redesign are generally necessary with analog components. Further, analog components require different grounds than digital components which creates additional unneeded complications. Therefore use of analog components in conjunction with digital components increases costs and is often undesirable.
Consequently, there is a need for a digital device to interface between different types of data transmission lines with the ability to utilize digital components to account and adjust for phase variances in data transmission clock speeds. There is a further need for a line card connecting an optical line and a network that may adjust and convert data transmission from one type of data transmission format to another so networks having different data transmission standards may intercommunicate without extensive reconstruction of data transmission equipment and lines.